I moved your post to the “Sensors” section of the forum since it seems more appropriate.
It sounds like you are referring to the “LEDON” pin on item #960. This pin is connected to a MOSFET that powers the IR LEDs. It is pulled-high on the board to enable the IR LEDs all the time, but you can drive it low when not taking readings to save power. Additionally, you could connect it to a high-frequency PWM to effectively control the LED brightness and decrease power consumption.
I am not sure what your question about the motion of the robot is referring to, but for reference, our Arduino library for the Pololu QTR Reflectance Sensors has support for locating a black line on a white surface and a white line on a black surface. You can find details about this in the library’s readLineBlack() and readLineWhite documentation.
If that does not answer your questions, could you try rephrasing them?
I think it will be better if you see this datasheet while studying the board. https://www.pololu.com/docs/pdf/0J12/QTR-8x.pdf